Internally illuminated signs have one or more translucent faces or message areas which are illuminated by lamps mounted inside the sign. While internally illuminated signs are often preferred over externally lighted signs, internally lighted signs of the prior art have been much more expensive to construct than externally lighted signs, especially when the sign is large. Internally illuminated signs are usually lighted with fluorescent lamps, although in some instances the light in internally illuminated signs is provided by mercury vapor or metal halide lamps. The translucent faces of the sign can be either rigid or flexible. Small rigid translucent faced internally illuminated signs often have hinged faces in order to provide access to the lamps for maintenance and relamping, whereas, large internally illuminated signs normally utilize flex faces and are built up to thirty-six inches deep to allow a person to enter the sign through the side and thereby provide access to the lamps for maintenance and relamping. In order to provide a sign having sufficient depth to allow human access, it is necessary for sign manufacturers to build a framework structure for the sign which adds significantly to the cost of the sign. In externally lighted signs, aluminum extrusions which are relatively inexpensive are often used as the outer framing. However, since aluminum extrusions have a practical cross-section limit of about fifteen inches, the depth of the large internally illuminated signs precludes sign manufacturers from using aluminum extrusions for the outside structure of the signs which require human access, thereby greatly increasing construction costs of internally illuminated signs of the prior art. In addition, in internally illuminated signs of the prior art, there has been difficulty in achieving the desired uniformity of luminance across the full area of the sign face, since there is a tendency for the portion of the face of the sign adjacent the lamp to be much brighter than the rest of the sign face.